This invention relates to a method and apparatus for non-impact printing in general and more specifically to printing with scanning light beams on laser-thermal dye transfer medium using pulse width modulation.
Pulse-width modulation, and pulse-number modulation, may be applied to various types of radiation sources used in non-impact printers: radiation sources incapable of emitting intermediate amounts of radiation, or radiation sources whose emissions do not attain consistent amplitudes; or radiation sources whose control is more easily or less expensively accomplished by pulse modulation rather than by amplitude modulation. Pulse-number modulation is described in Col. 2, Lines 9-19, of U.S. Pat. No. 4,375,064:
xe2x80x9cthe total optical energy applied to a picture element, i.e., the exposure H, is defined by the following expression:
H=Nxc2x7xcex94h
where xcex94h is the optical energy which is applied to a photo-sensitive material by the semiconductor laser in response to one high frequency pulse and N is the number of high-frequency pulses (pulse number) which are provided according to the level of an input video signal for the picture element.xe2x80x9d
Note that the original symbols E and xcex94e in this quotation have been changed to H and xcex94h, respectively, in order to conform to the nomenclature used in the remainder of this document. U.S. Pat. No. 4,375,064 modifies pulse-number modulation to produce pulse-width modulation in Col. 3, Lines 28-34, and its FIG. 3C by generating a single pulse whose duration of activation encompasses all of the pulses of an equivalent pulse-number modulated optical output. Even though the explanation of pulse-number modulation and FIG. 2 of U.S. Pat. No. 4,375,064 rely upon exposure in units of (energy per unit area) to attain a density level, the xe2x80x9cSummary of the Inventionxe2x80x9d states in Col. 3, Lines 52-54, that its object is xe2x80x9cto provide a signal with which a light beam is subjected to binary modulation to record an image having half-tonesxe2x80x9d, not continuous-tone images. The xe2x80x9cDescription of the Preferred Embodimentsxe2x80x9d specifies in Col. 4, Lines 25-29, that xe2x80x9cIt is desirable that the recording sheet . . . be a silver salt or electronic type which is capable of producing half-tones and is sensitive to the wavelength (red or infrared) of the semiconductor laser beam.xe2x80x9d U.S. Pat. No. 4,375,064 never mentions the important roles of the optical output""s irradiance profile on the image-recording medium and of the scanning speed in determining: the actual exposure deposited an any single location on that medium; and whether the resulting image is a halftone of binary density distribution or a continuous tone of many controlled density levels.
Pulse-width modulation is applied to laser-thermal imaging in commonly assigned U.S. Pat. No. 5,241,328 to improve xe2x80x9cthe linearity of the tone scalexe2x80x9d in the Abstract. Printing of an intermediate density is accomplished in Col. 2, Lines 46-51, by an xe2x80x9cLDCL circuitxe2x80x9d which xe2x80x9cimmediately drives the laser from a threshold near-on value to an optimum xe2x80x98full-onxe2x80x99 condition, and then leaves the laser full-on, for a time corresponding to the weighted digital value of that respective binary wordxe2x80x9d and then reduces the laser to a threshold near-on value by Col. 6, Lines 8-9, and Claim 1, sufficiently low to not transfer further colorant as inferred from the zero density for both the pulse-width modulated and the amplitude modulated tonescales at the lower right corner of FIG. 5 and Col. 8, Lines 10-12 of U.S. Pat. No. 5,241,328. The fact that the pulse-width modulated tonescale curve attains the same image density as the horizontally sloped saturation regime of the amplitude-modulated tonescale curve at high exposure (the upper left corner of the graph of the image-recording medium""s exposure response in FIG. 5 of U.S. Pat. No. 5,241,328) implies that the laser power during the pulsed exposures is great enough that an exposure lasting longer than the time for the writing spot to traverse its own full-width at half maximum attains the medium""s saturated density level. In the Abstract, the apparatus scans xe2x80x9ca finely focused spot of light from the laser along a linexe2x80x9d, but the linkage between the size of that spot and its scanning speed in determining whether a halftone or a continuous-tone image results is not discussed
U.S. Pat. No. 6,060,208 uses pulse-width modulation to produce the visual semblance of intermediate image densities by superimposing a rudimentary line halftone upon an externally supplied halftone image at a spatial scale finer than the lines per inch at which that supplied halftone is encoded. xe2x80x9cPulsing a laserxe2x80x9d exposes binary donors xe2x80x9ccreating tiny gap areas in the coverage of the colorant . . . much smaller than the screen dot . . . while operating in the saturation portion of the transfer function for the colorantxe2x80x9d according to the Abstract and reiterated in Col. 2, Lines 46-51, in Col. 4, Lines 37-47, and again in Claim 1, to produce intermediate densities less than would be produced by uniformly deposited colorant in the image. The experiment reported in Col. 6, Lines 18-28, notes, xe2x80x9cThe spot size of the laser or other energy source used to transfer colorant from a donor to the substrate is typically a significant fraction of the area of each pixel. Therefore, one cannot simply turn off the laser (or other energy source) for 2% of the time to produce the desired apparent optical density of the deposited colorant. Varying duty cycle of a laser in a laser color proofer in the range of about 50% to about 80% can provide a useful range of apparent optical densities in some cases.xe2x80x9d U.S. Pat. No. 6,060,208 does not teach a reason why the spot size of the laser precludes use of a 2% off-pulse to reduce image density, or how to predict the actual duty cycle that would be required to produce a desired exposure or density. The consequences of scanning speed for pulse duration and beam size are not discussed.
The first three embodiments of commonly assigned U.S. Pat. No. 5,874,981 are specifically stated in Col. 4, Lines 38-45, and references to Ttraverse, to complete exposure deposition by one pulse of the source in less time than its beam requires to traverse its own width on the donor. The second three embodiments encompass pulse durations longer than Ttraverse. U.S. Pat. No. 5,874,981 uses shaped pulses in addition to simple binary pulses of the exposing light to exploit intrinsic characteristics of laser-thermal dye transfer to obtain desirable tonescales in the consequent images while simplifying the electronic control of the light sources in Col. 2, Lines 34-38. Neither reproduction of desired exposure profiles nor adjustment of exposure deposited by multiple sources are stated as objects of this patent.
Multiple-source printers produce better image quality and reduced artifacts when all of the sources are matched to deposit identical exposure profiles generating identical image densities with identical spatial extent. This matching of exposure profiles deposited by all sources in a multiple-source printhead is sometimes called xe2x80x9cprinthead balance.xe2x80x9d Imbalance of a printhead can produce undesirable streaking artifacts in its printed images because some scanlines are darker than others. The mechanism of creating some scanlines darker than others on a continuous-tone image-recording medium might be simply the difference in that medium""s response to some sources emitting different powers than other sources in that printhead. Commonly assigned U.S. Pat. No. 5,266,973 measures the powers of individual sources in the printhead in response to a sequence of electrical currents applied during calibration, then rescales the currents encoding the image sent to each laser, eliciting the same power from each of the sources in order to impose balance during printing. Balance is attained when all of the lasers are adjusted to produce the same image densities over the same spatial extent in the image-recording medium within an acceptable tolerance. Commonly assigned U.S. Pat. No. 5,291,221 and commonly assigned U.S. Pat. No. 5,323,179 apply a sequence of electrical currents while exposing the intended image-recording medium, then measure the image densities produced on that medium, and rescale the currents encoding the image to elicit the same image density during printing.
Printhead imbalance can produce streaking artifacts in halftones, even when printing on binary image-recording medium that produces one density when the exposure remains below an imaging threshold and a second density at any location receiving exposure exceeding that threshold. As illustrated in FIG. 1 for two exposure profiles deposited on a binary image-recording medium by the same irradiance shape but produced by twice the power from the source on the left as on the right, the lateral extent of the exposure profile exceeding the density-change threshold is wider for greater source power. The local density at the center of the exposure profile deposited by the lower power is the same as the local density at the center of the exposure profile deposited by the higher power due to the binary response of the image-recording medium. But since the lateral extent of the density change increases for the higher power, and the lateral extent of unchanged image density decreases, the density corresponding to the area-averaged transmittance or reflectance of an imaged area wider than the distance between successive pulsed exposures produced by the higher powered source differs from that produced by the lower powered source.
U.S. patent application Ser. No. 09/283,068 for the common assignee entitled xe2x80x9cModulator for Optical Printingxe2x80x9d states in Lines 14-21 of Page 11:
In a further feature of the present invention, the amount of light deposited by each pixel at the media plane can be controlled, and therefore balanced, by adjusting the pulse width of the voltage applied to each individual pixel during a line printing time. There would need to be a calibration of the pixels before printing in order to determine the pulse width for each pixel. In this way, a more uniform printing by the entire array of pixels would result. This provides a means to correct for reasonable non-uniformities in the illuminations, the electrodes, the coatings and the crystal.
Its Claim 10 states:
A modulator according to Claim 1, wherein a pulse width of a voltage applied to each pixel of said electrode is adjustable to balance light levels during printing.
U.S. Pat. application Ser. No. 09/283,068 does not distinguish between pulses of shorter duration than the time for a writing-spot center on the image-recording medium to traverse its own full-width at half maximum (FWHM) and pulses longer than that FWHM traversal time. If the xe2x80x9clight levelsxe2x80x9d, i.e., instantaneous powers emitted by the light sources, are not adjusted in amplitude but only binary modulated, balancing the deposited exposure among multiple sources in a printhead depends upon the relative duration of the pulse compared to the FWHM traversal time.
It is desirable to provide a printer capable of producing an amplitude modulated exposure profile by applying binary modulation to a radiation source. It is desirable to produce an intermediate density on an image recording medium exhibiting a continuous-tone density response to exposure, such as photograghic film. It is desirable to transfer intermediate amounts of colorant from a laser-thermal donor. It is also desirable to provide this capability while avoiding undesirable gaps in the image density profile. It is also desirable to provide a means for controlling the width of exposure profiles and consequently image tracks by pulse width modulation of a radiation source using pulses shorter than FWHM transversal time. It is further desirable to match the exposure profiles produced by a plurality of sources in a multiple-source printhead by applying binary modulation to those sources.
Briefly, according to one aspect of the present invention a method of controlling exposure of a medium comprises the step of changing an emission level of a radiation source from a first power to a second power. The second power emits for less time than required by an irradiance profile to traverse a full-width at half maximum of the irradiance profile projected onto the medium along a direction of relative motion between the irradiance profile and the medium. The emission level changes to the first power and emits less time than required by the irradiance profile to traverse the full-width at half maximum of the irradiance profile projected onto the medium along the direction of relative motion between the irradiance profile and the medium, then changes the emission level to the second power.
The irradiance profile projected by the light source onto the image-recording medium can reproduce a desired exposure profile in that image-recording medium by extinguishing that light source before the exposure reaches its infinite-duration limit H[toff-tonxe2x86x92∞]. That infinite-duration limit for the exposure is determined by the light beam""s irradiation profile on the image-recording medium and by the amount of time required for any point on that beam""s irradiance profile to scan a distance on the image-recording medium equal to that irradiance profile""s full width at half maximum. This ability to enable a binary light source to imitate an amplitude-modulated source can be exploited in apparatus needing adjustability of the apparent amplitude and width of the light source, such as a multiple-source printhead with power variation among sources.
When a light source is pulsed to emit a constant power for a duration shorter that the time required for the irradiance profile to traverse its full width at half maximum (FWHM) along the scanning direction projected by that light source on an image-recording medium, the amplitude of the local exposure deposited on that image-recording medium is controlled by the duration of that pulse. Brief pulses of a binary-modulated source can deposit exposure profiles on the image-recording medium indistinguishable for the purposes of imaging from those that amplitude modulation of that light source could have deposited. Pulsing of a single binary source for durations shorter than the irradiance-FWHM traversal time can deposit graded amounts of exposure, producing intermediate image densities on a medium exhibiting a continuous-tone transfer response. Transfer of intermediate amounts of colorant from more than one laser-thermal donor can be accumulated in dot-on-dot printing described in commonly assigned U.S. Pat. No. 5,309,246 to produce halftone dots of xe2x80x9cspecialxe2x80x9d colors differing in appearance from the color obtained with a single donor. This capability of pulse-width modulation to control the amplitude of the deposited exposure can be used to adjust multiple binary sources emitting different powers in a printhead in order to cause them to deposit equal exposure profiles by pulsing each source for the compensating duration, while avoiding an artifactual gap in the image density of each subpixel.
One advantage of this invention over prior art is the capability of producing an amplitude modulated exposure profile by only applying binary modulation to an irradiation source, while avoiding undesired gaps in the image-density profile that might otherwise be needed to produce graded image density by means of halftoning on a spatial scale finer than the pixels of the original image. A second advantage of this invention is the ability to control the width of exposure profiles and consequent image tracks by pulse width modulation of a binary-modulated source using pulses shorter than the irradiance-FWHM traversal time. In a multiple-source printhead, binary modulation to equalize the exposure profiles offers the possibility of reducing the number of connections and simplifying the control devices for binary modulation, and enables the possibility of exposure balance for printheads incapable of amplitude modulating some sources independently of the other sources. The relatively wider irradiance profile full-width at half maximum specified for amplitude modulation of the exposure profile by binary modulation of the source is usually easier to attain in a printer and exhibits larger depth of focus than the narrow irradiance profile required for producing halftones with gaps in image-density change within each subpixel of the original image.
The present invention exploits the recognition of two regimes of exposure profiles available by pulse-width modulation of a binary modulated light source: contone and halftone. The criterion for demarcating these two regimes is the relative duration of the pulse compared to the time for the irradiance profile on the image-recording medium to traverse its own full-width at half maximum. This criterion can be restated in the distance domain as the relative distance scanned by the center of the irradiance profile on the image-recording medium during the pulse compared to the irradiance profile""s full-width at half maximum. Wider beams and briefer pulses are used to adjust the amplitude of exposure profiles and the width of image tracks than would be required for halftoning on a scale finer than the printer""s subpixels. Image-density gaps finer than the printer""s subpixels are not needed to improve uniformity of the printing with a binary modulated radiation source.
Maintaining the pulse width shorter than the irradiance-FWHM traversal time can control the width transverse to the scanning direction of a region in which the deposited exposure exceeds a threshold exposure required by an image-recording medium to undergo a transformation; track width cannot be modified significantly by modulation in the halftone regime with pulses longer than the irradiance-FWHM traversal time.
The present invention is also useful in printing on silver-halide film and paper; and in illumination for input-scanning readers; and in integrated circuit manufacture, more specifically in ablative transfer of chemicals for doping or depositing upon a semiconductor or other substrate. A specific application is described for multiple-source printers or illuminators.